The present invention relates to optical polarization sensing apparatus and methods that are employed to monitor the change of the polarization state of light to detect and sense signals.
Optical polarization sensing devices have numerous applications, including stress, pressure and temperature sensors, for example. Typically, these devices are fairly large and employ expensive components to achieve a required level of sensitivity. For example, there have been extensive studies conducted on developing non-invasive blood glucose monitoring for diabetes patients because of the inconvenience and danger of the conventional invasive methods, which usually require the patients to cut a finger to give a blood sample. By shining light through human interstitial fluids or blood at translucent dermal positions such as fingertips, ear lobes, or ocular aqueous humor in the eyes, information about the glucose levels can be obtained optically by infrared spectroscopy, Raman spectroscopy or optical polarization changes. Because glucose rotates a polarized light beam's polarization proportionally to its concentration, a phenomenon known as optical activity, various polarimetric glucose sensors have been developed. Examples of these include devices that use Faraday rotators to directly modulate the beam's polarization state, as well as devices that use a Zeeman laser for optical heterodyne detection. However, these methods involve bulk optical components that are expensive and inconvenient.
A need therefore exists for inexpensive, small scale, high sensitivity polarization sensing devices, particularly in certain applications, such as the aforementioned detection of blood glucose levels. In this application, small size and low cost are important to enable the devices to be both portable and affordable so that they may be purchased and used by the diabetic patients without having to visit a medical facility. Another application of these devices is in a magneto-optical disk read head.
One known technique for fabrication of ultra-small micromechanical structures and devices is a polysilicon surface micromachining technique known as MEMS (Micro Electro Mechanical System). With MEMS technology, micro-sized mechanical actuators, sensors and other structures can be integrally formed on single silicon substrates or chips with integrated circuits that control, or receive signals from, the actuators or sensors. To date, however, polarization sensing devices have not been formed using MEMS technology. The necessary sensitivity of the polarization sensing systems, as small as 0.1 to 0.001 degree change in polarization rotation angle, is very hard to achieve with conventional direct detection methods. The conventional light detection schemes can not meet the sensitivity requirement without going through very sophisticated optical systems, which would be much too bulky and expensive, and involve components such as Faraday rotators that are not compatible with the MEMS technology.